Power cables having an operating voltage of 6 kV or above typically comprise inner and outer conductive layers in the cable insulation for field equalization. In order to connect such cables with other cables or devices, individual elements of the cables are in the area of the cable ends gradually exposed or cut back, respectively, and enveloping devices or connecting devices known as cable fittings such as e.g. cable joints and cable terminations are arranged at the cable ends. The existence of a cut-back end of an outer conductive layer, leads to an increase in the electric field at this position in the operation of the respective cable. In order to guarantee a safe operation and in particular to make sure that the increase in the electric field does not lead to discharges, breakdowns or flashovers, the electric field is graded or controlled by suitable means and measures which are known as field control. In this respect, known cable fittings usually comprise corresponding field-control elements or field-control bodies.
High voltage direct current (DC) cable joints comprise typically a multi-wall structured tubular geometry (see FIG. 2) consisting of different kinds of material grades. The arrangement of the individual material layers and their design are selected in a way, that the core (cable) of the joint is insulated from the ground potential (outside) at normal operating conditions, and that field grading properties are achieved for prevention of local electrical field enhancement. See e.g. WO-00/74191, and WO-2007/147755.
WO-00/74191 relates to a device for controlling an electric field at a connection or a joint of a high-voltage cable for DC voltage. The electric field control is achieved by geometrical field control.
WO-2007/147755 also relates to a device for electric field control. The device has a layered structure comprising a resistive layer for field control, an insulating layer arranged on the resistive layer and a semi-conducting layer or conducting layer arranged on the insulating layer. The manufacturing of the device includes the steps of winding the resistive layer on a carrier and then grinding to a desired shape; winding the insulating layer outside the resistive layer and grinding it to a desired shape, and further on with the remaining layers.
Joints are often manufactured from EPDM (Ethylene Propylene Diene Monomer) rubber in multiple process steps. EPDM rubber is a type of synthetic rubber and is an elastomer which is characterized by a wide range of physical properties.
In the multiple process steps layers of different uncured EPDM rubber types are wound around a mandrel and cured sequentially under elevated temperature and under pressure in a surrounding steel mould (compression moulding). The individual material layers of such cable joints consist of material with different electrical properties. In a typical cable joint, which is illustrated in FIG. 2, there is (from inside to outside) a conductive layer 1 which is on high voltage potential, a field grading layer 2, an insulating layer 3 and a conductive layer 4 on the outside which is on ground potential. The field grading layer 2, consisting of EPDM rubber containing filler materials leading to the appropriate nonlinear electrical behaviour, has two main functions: Primarily it controls the electric field from the non-insulated cable ends at normal operation voltage. Secondly, it is designed to build-up space charges which supress local field enhancements in the case of over voltage, in order to protect attached electrical devices.
Following the above described manufacturing method and design recommendation, on an industrial scale, DC cable joints are produced in the range of 80 kV to 320 kV.
The advantages of these manufacturing techniques are e.g. that they are well-established, and that the process for low voltage ratings is efficient.
However, the current manufacturing method related to the processing of EPDM and the EPDM curing chemistry may be labour-intensive and thereby costly. One reason is that EPDM is already in the uncured stage a very highly viscous material and, as discussed above, is often wound in form of uncured EPDM wounds around a core (mandrel), or respectively around the precedent layer of the semi-finished joint. Subsequently, it is compression wound and cured under elevated temperature.
EPDM is cured by a radical curing reaction, initiated by peroxides as curing agents. During the curing step, the peroxides are decomposed into low molecular, volatile compounds. These by-products may negatively influence the electric properties of the EPDM rubber. Therefore, the cured EPDM cable joint has to be degassed to remove these volatile by-products.
U.S. Pat. No. 5,801,332 relates to an elastically recoverable silicone rubber splice cover comprising layers of insulating, semi-conductive and conductive silicone rubber, and in particular platinum catalyzed silicone rubber. The cover is produced by e.g. using injection molding technique.
US-2005/0139373 relates to a sleeve for a high-voltage cable and cable element provided with such a sleeve. The sleeve comprises a field control element made from a field strength-dependent material, e.g. silicone rubber, EPDM or natural rubber, by e.g. using an injection-molding process.
EP-2026438 and EP-2019466 relate to cable connection electric field control devices comprising an insulating body and a field-grading layer embedded in that body. Typical field grading materials that form the field grading layer may be silicone rubber or EPDM filled with semiconducting material particles.
The general object of the present invention is to achieve an improved high-voltage DC cable joint having optimal electrical and mechanical properties. And in addition an object is to achieve an improved manufacturing method resulting in an improved high-voltage DC cable joint where the degassing of volatile by-products is avoided.